Hip joint forces in hip replacement patients and normal subjects during activities of daily living
A high number of revision hip replacement operations are currently performed due to loosening of the primary implant. The loading imposed on the prosthetic joint and its fixation mechanisms may be one of the many factors contributing to the loosening process. Previous work to determine hip joint loading has concentrated on gait, stair negotiation and rising from a chair. However, since patients often comment on the difficulty of getting into and out of a car and bath, these activities are also included in the current project. The 3 orthogonal components of hip joint force have been calculated for 16 postoperative hip replacement patients between one and two years after surgery and also for 10 age-matched normals. A biomechanical model of the lower limb was developed including 37 muscle elements. Algorithms were incorporated to correct for curved muscle paths, providing realistic muscle moment arms with changing joint angle configuration. An optimization routine which minimizes the o verall maximum muscle stress was incorporated to determine muscle forces which were then used in the calculation of joint force. The model utilizes anatomical muscle and bone data, kinematics measured using a 6 camera Vicon motion analysis system and ground reaction forces measured using force platforms. In validity tests, the predicted muscle activity patterns for normal subjects were found to be consistent with published EMG data for most muscles. The mean peak resultant hip joint force of 3.8 times body weight calculated for the patients during gait at 1.01 m/s was consistent with the results published for patients with instrumented hip prostheses at a measurement time of more than 12 months after surgery. The maximum mean peak resultant hip joint force determined for patients was 5 times body weight, calculated at the left hip when getting out of the passenger side of a right hand drive car. A simple calculation of torsional moment about the stem of the femoral component during this and other activities showed it to be close to or to exceed the experimentally determined limits of torsional strength of implant fixations, reported in the literature. The maximum mean peak resultant hip joint force calculated for normals was 6.3 times body weight, determined at the left hip on getting into the passenger side of a right hand drive car. It is suggested that car entry and exit and other activities should be performed in safer styles rid that the results of this thesis should be incorporated into the design and testing of hip prostheses.